Class D amplifiers, which are sometimes also referred to as switching amplifiers typically use two serially connected power transistors such that the first of the power transistors switches the output to the positive voltage supply whereas the second of the power transistors switches the output to the negative voltage supply. The power transistors are typically controlled by a pulse width modulated version of the audio input signal. Class D amplifiers are interesting devices in low-power applications because of their high efficiency compared to traditionally used linear AB-class amplifiers.
A previous drawback of class D amplifiers was that in order to extract the output signal from the amplified modulated signal, a relatively bulky low pass output filter was required, thus for instance limiting the applicability of class D amplifiers in compact designs. However, P. Muggler et al. disclosed in Proc. Int. Symp. Circuits and Systems, Vol. 1, May 2004, pages 1036-1039 that such a filter could be omitted from the design of the class D amplifier by the use of a bridge-tied load configuration in combination with 3-level pulse width modulation (PWM) scheme, and using the low-pass behavior of the magnetic speaker as the filter.
Consequently, class D amplifiers have gained considerable interest as low-power loudspeaker drivers in for instance mobile communication applications as they have a far better power efficiency than traditionally used class AB amplifiers.
Nevertheless, design challenges still have to be overcome to ensure that a class D amplifier operates as efficiently as possible. In order to optimize signal amplification, the on-resistance (Ron) of the power transistors has to be minimized. This can be achieved by increasing the size of the power transistor. However, the consequence of such a size increase is that the idle dissipation of the power transistor stage is increased, thereby reducing the efficiency of the class D amplifier and reducing battery life if the amplifier is battery-powered. The charge and discharge current of the power transistor gates forms a large portion of this dissipation, and scales with the size of the power transistor. Consequently, one of the main design challenges in class D amplifier design is to find a suitable trade-off between maximum audio output power and maximum idle dissipation.